Fix empty $path triggering a warning, and failing on Windows.

Slash terminating a path requires reading the last char of the path,
which will fail on an empty string. Also on Windows the assumed
top level path should use the proper backward slash.

src/Filesystem/Folder.php

@@ -418,12 +418,17 @@ public function inCakePath($path = '')
     /**
      * Returns true if the Folder is in the given path.
      *
-     * @param string $path The absolute path to check that the current `pwd()` resides within.
+     * @param string $path The absolute path to check that the current `pwd()` resides within. If omitted
+     *  (or empty), the current top level directory is assumed.
      * @param bool $reverse Reverse the search, check if the given `$path` resides within the current `pwd()`.
      * @return bool
      */
     public function inPath($path = '', $reverse = false)
     {
+        if (empty($path)) {
+            $path = DS;
+        }
+
         $dir = Folder::slashTerm($path);
         $current = Folder::slashTerm($this->pwd());